1. Field of the Invention
The present invention relates to an ultrasonic probe, particularly, a medical ultrasonic probe for viewing or imaging sections of a body with a scanning of an ultrasonic beam by a reflecting mirror.
2. Related Art Statement
As a conventional ultrasonic probe of a mechanical radial type, there is a system for scanning the section of the body by ultrasonic energy in such a manner that an ultrasonic transducer is rotatably provided on the tip portion of the ultrasonic probe, to scan it mechanically while rotating the ultrasonic transducer, thereby rotating and scanning it over 360.degree., or a system for scanning a plane mirror over 360.degree., in which the ultrasonic energy radiated from an ultrasonic transducer, such as, for example, piezo-electric element, is reflected by a plane mirror (for example, refer to Japanese Patent Application Laid-open NO. 218,144/92).
Such an ultrasonic probe is used in a system apparatus having an image display means or the like.
An ultrasonic probe sector-scanned by swinging it mechanically is disclosed in Japanese Patent Application Laid-open NO. 104,339/90.
It is advantageous that these ultrasonic probe are provided with means for making the radiated ultrasonic beams thinner in order to view areas to be viewed of a subject body with high resolution.
It is ideal to obtain the ultrasonic probe in such a manner that a uniformly thin ultrasonic beam is formed. However, since it is technically difficult to form such an ideal ultrasonic probe, an acoustic lens is attached to the surface of the utilized transducer, or a curved mirror is used to obtain the ultrasonic beam with high resolution by focusing the ultrasonic wave on the areas to be viewed in the ultrasonic probe combined with the above system device.
However, if a concave acoustic lens or a concave mirror is utilized separately, an excellent image can be obtained with high resolution at focal point of the ultrasonic beam, but the ultrasonic beam becomes thick at its off-focal points, thereby decreasing resolution.
In the case of an ultrasonic endoscope, particularly, if the areas near the center of the endoscope are to be viewed with high resolution, the curvature of the acoustic lens or the concave mirror for the ultrasonic transducer is made small, thereby setting the focus of the beam near the probe.
FIG. 15 is an explanatory view showing the conditions of such an acoustic beam, in which the width of the acoustic beam is plotted to the distance from the transducer surface. That is, an abscissa is plotted as the distance from the transducer surface, and an ordinate is plotted as the width of the acoustic beam. As shown from FIG. 15, if the focus becomes near, minimum beam diameter becomes comparatively small, but the focal depth becomes shallow, so that the resolution becomes inferior without near the focus, and thus the sensitivity becomes low, thereby decreasing the image quality remarkably.
As seen from the above result, even if the acoustic lens and the concave mirror for the transducer are only accommodated in the ultrasonic probe as a constituent element, and set as described above, the diameter of whole beam is thin, so that the ultrasonic probe having high resolution which is not influenced by the distance from the probe can not be obtained.
If the areas far from the center of the endoscope must be observed with high resolution, the curvatures of the acoustic lens and the concave mirror are made large and the focus thereof is set far in the distance.
The characteristic diagram exhibiting the shape of the acoustic beam is shown in FIG. 16. In the case of this embodiment, if the focus is situated in far distance, the diameter (thickness) of the ultrasonic beam in comparatively far distance becomes substantially unity, but the diameter of the minimum beam in focus point can not be made small, thereby obtaining an image having substantially inferior resolution.
In this case, an improving method at the side of ultrasonic probe has a certain limit, so that the ultrasonic probe capable of observing the subject body with high resolution irrespective of the distance from the probe near distance to far distance thereof can not be realized.
The method of widening the focus depth with various technics has been tried, but this method require the change or modification of the system, and can not be realized easily. That is, any method requires large and complicated electric circuit, and thus large ultrasonic transducer is required, so that this method can not be realized easily in the ultrasonic probe.